#include<stdio.h>
int main()
{
	int count=0;
	int number;
	int sum=0;
	scanf("%d",&number);
	while(number!=-1)
	{
		count++;
		sum=+number;
		scanf("%d",&number);
	}
	printf("%f",1.0*sum/count);
	return 0; 
 } 
